Composite porous solid articles, such as porous separation articles and carbon block filtration articles, are known in the art. These articles are produced using mixtures of thermoplastic binders and active particles or fibrous materials such as activated carbon powder. The articles preferably are formed under conditions effective to permit the thermoplastic binder to connect the active particles or fibrous materials in discrete spots, rather than as a coating, forming an interconnected web. This arrangement permits the active powdery or fibrous materials to be in direct contact with, and to interact with, a fluid or gas. The resulting composite solid article is porous, thereby permitting the fluid or gas to penetrate into and pass through the article. Such articles are especially useful in water purification, purification of organic waste streams, and in biological separations.
U.S. Pat. No. 6,395,190 describes carbon filters and a method for making them having a 15 to 25 weight percent of a thermoplastic binder, where the average particle size is from 5 to 25 microns; and having activated carbon particles where the majority of the particles are in the 200-325 mesh range (44-74 microns), with the rest of the activated carbon is less than 325 mesh.
Poly(vinylidene fluoride) as a binder for a porous block article, has been found to improve the performance of the article by providing effective binding at lower loading—which in turn opens more of the surface of the active agent (like activated carbon, zeolites or ion exchange agents)—providing even greater efficiency.
Examples of such composite porous solid articles, as well as methods for producing them, are described for example in WO 2014/055473 and WO 2014/182861, the entire disclosures of each of which are incorporated herein by reference for all purposes. These articles use polyvinylidene fluoride or polyamide binders, rather than the polyethylene binders previously used for carbon block filtration articles.
In some applications, it is desired to add additional functionality to the porous block article, such as the addition of an antimicrobial agent. An agent, such as AgBr has been added to a porous block article, as described in U.S. Pat. No. 7,144,533 and US 2011/0210062. Each of these references describes a complicated method for adding a metal (preferably Ag or Cu) salt into a filtration device, involving first charging the activated carbon or binder, then adding a charged microbiological interception agent. AgBr can be added at about 0.05 to 0.5%, with a particle size of about a micron.
There is a relationship between the level of loading of particulate agents, the size of the agent, and the available surface area of the agent. It is desired to have a large amount of surface area of a particulate agent available, to maximize effectiveness as a filtration/active agent. Small functional particles provide the most surface area—and thus the highest distribution efficiency per weight of agent by volume of filtration article. Yet, the smaller the particle, the more of the binder surface will be clogged with particulate agent—meaning less binder surface is available to actually bind the primary particles. In the art this is handled by either using large functional particles (generally 5 microns or larger)—about the same size as the primary particles like activated carbon), or else nano-sized functional additive particles could be used, but only a low levels (far less than 1% and preferably less than 0.5%). Alternatively, a higher level of binder could be employed, but the more binder, the less surface area of the primary particles is available for filtration. Many of the functional additives are available in nano-size—having large surface areas available for reactions.
The problem solved by this invention is to add higher loadings of functionalization to a solid porous separation article, while maintaining sufficient mechanical strength to prevent collapse of the article during a pressurized fluid flow.
Surprisingly, it has now been found that by using discrete polymer binder particles of 50 to 500 nm, high loading (>0.5 wt %) of nano-particles can be achieved, while the binder remains effective to bind the primary active particles. The nano-particles achieve good dispersion